Improve Fiddle::Pointer#inspect for FFI backend (#179)

```ruby
p Fiddle::Pointer.new(1)
```

CRuby

```text
#<Fiddle::Pointer:0x0000600000983240 ptr=0x0000000000000001 size=0 free=0x0000000000000000>
```

JRuby before

```text
#<Fiddle::Pointer ptr=1 size=9223372036854775807 free=nil>
```

JRuby after

```text
#<Fiddle::Pointer ptr=0x0000000000000001 size=9223372036854775807 free=0x0000000000000000>
```

Note: `size` is incorrect for JRuby, which is not addressed with this PR

---------

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

Author: ankane

lib/fiddle/ffi_backend.rb

@@ -433,7 +433,16 @@ def to_value
     end
 
     def inspect
-      "#<#{self.class.name} ptr=#{to_i.to_s(16)} size=#{@size} free=#{@free.inspect}>"
+      # SIZEOF_VOIDP * 2 == Math.log(2 ** (SIZEOF_VOIDP * 8), 16)
+      pointer_inspect_width = SIZEOF_VOIDP * 2
+      "#<%s ptr=0x%0.*x size=%d free=0x%0.*x>" % [
+        self.class.name,
+        pointer_inspect_width,
+        to_i,
+        size,
+        pointer_inspect_width,
+        @free || 0,
+      ]
     end
 
     def +(delta)

test/fiddle/test_pointer.rb

@@ -110,10 +110,6 @@ def test_plus
     end
 
     def test_inspect
-      if ffi_backend?
-        omit("Fiddle::Pointer#inspect is incompatible with FFI backend")
-      end
-
       ptr = Pointer.new(0)
       inspect = ptr.inspect
       assert_match(/size=#{ptr.size}/, inspect)